Problem:

Calculating the average of the values in every 4th line.

Solution:
Use the MOD and ROW function to find every 4th cell in your list.
Calculate the average of those numbers using this array formula:
{=AVERAGE(IF((MOD(ROW($B$2:$B$10)-11,4))=0,$B$2:$B$10))}


Serial Number___Value
1_______________20
2_______________40
3_______________50
4_______________30
5_______________8
6_______________400
7_______________35
8_______________10
9_______________100

Result: 20